For measuring displacement in motor vehicles, inductive displacement sensors may be used since in addition to their robustness to ambient influences such as changes in temperature, shaking stress and soiling they are also relatively cost effective to manufacture.
If electromagnetic components, for example solenoids, are used in the vicinity of such an inductive displacement sensor, the sensor is frequently subjected to an interfering influence when a solenoid valve is actuated.
There are various ways of minimizing these disruptive measuring errors of the sensor.
Sensors may be frequently surrounded with a metallic screen sleeve in order to screen off external electromagnetic alternating fields and reduce them to an acceptable degree. However, these screen sleeves make the sensor larger and more expensive. While the sensors are composed of a coil and a plastic injection-molded sheath surrounding them, and thus have a low intrinsic weight, the screen sleeves are manufactured from a comparatively thick and thus heavy steel casing.
These screen sleeves are frequently fabricated from material which is difficult to work and thus also increases costs.
German patent document no. 100 51 048 A1 discusses a method of preventing such disruptive influence of external magnetic fields on an inductive sensor by virtue of the fact that the time when the sensor value is read in is matched to the actuation times of the solenoid valves in such a way that the measuring time of the sensor always lies outside the actuation times of the solenoid valves.
However, this may only be possible if there are sufficiently frequent and correspondingly long times of rest between the actuations of the solenoid valve in which periods the sensor value can be registered. This results in disadvantages in terms of the dynamic tracking speed when there are rapid changes in sensor value.
In addition to the influence from electromagnetic fields, interference from electrical fields acting on the inductive displacement sensor is also possible.
German patent document no. 43 18 263 discusses a method and a circuit for temperature-compensated starting up of a plurality of predefined positions. The core idea of this patent is that for precise position control the displacement sensor which is used is not compensated but rather conversely the anticipated sensor value for the set point positions which are to be adopted are calculated in advance in accordance with the respectively prevailing temperatures and then positioned to this value.